I'm probably looking for something in the 12" range but please toss out some inexpensive options and we'll go from there.

Also, is there an optimum sbr length for x39?

kak industries has some barrels in 8 inch , 11 inch and 16 inch and it looked like they had bolt carrier groups . give them a call and tell them what you want . kurt is a nice guy and will help you out . I know alot of 300 blackout guys who have their 9 inch 300 barrel and are pleased with them . I ordered some bullets from them for a caliber not related to this discussion and shipping was fast and the product was excellent .

10.5" Barrel here at Tactical ambush.  I've got one of their 16" 5.56 barrels on another rifle and it's worked well.  For $120 it might not be a bad buy it/try it option.  I'm about to build a 7.62x39 pistol with Sig brace after I finish my Aero .308 rifle, and this is mighty tempting for a barrel, for me at least.  Definitely love my 7.62x39 AR, I shoot it more than anything else in the safe.

Thanks,

Is there a "best" SBR length for x39?
I have this, probably unfounded, voice in my head saying 10.5" is too short...

you'll be fine with a 10.5 barrel

123 grain projectile
2400 from 20"
2350 from 16"
2250 from 12.5"
2100 from 10.5"
2000 from 9"

Glad you had luck with yours. I'm still trying to get the first of four 10.5 TA barrels to work correctly. Severely under gassed.

I'm seriously considering a 7.62x39 SBR, and if I do, I'll probably go with an 11.5" to 12.5" tube.  Given the 10.5" has such short dwell time and the '39 is a lower pressure and larger bore than the 5.56, not to mention some variations in loads, I'd feel a little more confident running a slightly longer barrel to give me a little more dwell.  The fireball out of my AMD-65 pistol was pretty significant, and it had a 12.5" tube, so I wouldn't want to go much shorter than that for the sake of efficiency.

Good info. I was leaning towards 13"ish

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Also, $178 for a Cason bolt sounds steep. Are there other options?

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It is the best and has been proving itself lately with high round count from other posters here. Other bolts are known to fail early.

I am using an 11.5" YHM barrel and Cason BCG on my setup. Has been great so far.

Is there any confirmation on the "newer" bolts failing? Older bolts were 8620 while new ones are 9310. I know nothing about metallurgy and only a little bit about the strains a 7.62x39 BCG under goes due to the design of the bolt and the stresses the caliber puts on it.

Complete 7.62 x 39 bolt. Bolt is Made from 9310 steel
http://www.maximfirearms.com/online-store/complete-7-62-x-39-bolt/

If you aren't running a can, you probably need this:  

 









 

I think I'll be using an AAC Single chamber brake

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7.62x39 AR's are not your run of the mill home build projects where you can slap parts together and all is well.

There are some guys that know what they are doing, but not many.  Bolts, extractors, and correct bore diameter are the killers, as well as variation in charge weights lot to lot and company to company.

Gassing them hard brings with it problems with the operating system, which needs to be mitigated with recoil spring, buffer, and carrier weight so you don't destroy extractors, almost all of which are made to improper dimensions for the extractor rim thickness of the Russian short.

It really takes a systemic approach to assemble a reliable and long-lasting 7.62x39.  I know a bunch of you will pipe in that your parts basher has been running 100% for thousands of rounds, but the list of actual high volume guns' trail of tears with broken extractors and broken bolts is very, very long.  

Your thoughts on a 16" surplus arms and ammo, carbine gas chopped to 12.5" with the above maxim bolt? I've also been recommended a Soto bolt.
Can I buy a cheap spikes/PSA/ares bolt carrier?
How about buffer and spring?
Honestly, I'd be impressed if the gun saw 750rnds a year
Please and thank you

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I agree that any other caliber than 5.56 builds require a fair bit of tinkering, my 5.45x39 build was horribly unreliable until I tuned the bolt and magazine, once that was sorted out she ran like a champ for thousands of round with zero malfunctions. My recent 7.62 build ran perfectly but had sharp recoil with the stock carbine system. well, with my new batch of wolf ammo I received, I started getting flattened primers and then FTE's. This is a good example of lot to lot inconsistencies with imported steel cased ammo. This ammo is really hot compared to my other lots, some of which appear to have come out of the same factory 30.  these things are very easily overcome with adjustable gasblocks and x-power springs and buffers. I think most of the broken bolt issues have been fixed with proper metallurgy, not many have failed that have been built in the last few years.as for the 5.56 extractors failing, as long as it lasts a couple thousand rounds and is reliable, I'd rather have a bolt that uses the 5.56 extractor since I have them on hand as a wear item, the companies that build 7.62x39 parts seem to come and go so I like parts commonality. I shoot a lot and lately I've been going through 500 rnds of 7.62 a month, I'll certainly let yall know if any other issues arise with mine after doing the extractor upgrade (fixed my ejection issue) along with the new xp spring and adjustable gas block. I really enjoy other caliber builds, it is fun being able to id and fix these issues and assemble a reliable weapon.

The bolt carrier doesn't matter as long as it's in spec.  After my recent experiences with mine I'd opt for an adjustable GB a H2 buffer and xp spring right off, with the adjustable GB being the most important of the 3. I don't know how the short barrel would affect things though.

With a short barrel, you are going to need to gas it hard with a larger port, which then steers you into faster cyclic rate, which then becomes problematic for extraction.

One buffer system that I have had very pleasant results with is the MGI Rate Reducer and Recoil Buffer, which is heavy, but with a springed mechanical internal reciprocating arrangement that really tames guns down.

With the AR family, we are looking for that happy medium balance where the operating system hums along at a comfortable cyclic rate, and that is really hard to achieve consistently with 7.62x39 steel cased ammo.

I didn't even address magazines with this caliber, as there are plenty of hurdles alone in the critical components of the gun itself that have to be tackled.

If I were building a 7.62x39 carbine right now, I would look at the work Colt did carefully, and take it from there.  Those Colt Sporter 7.62x39 carbines are worth looking over in detail.

As to magazines, most people install an AK mag spring in the ASC mags to increase the spring strength.  With the variant calibers in the AR, a lot of assume that the same testing and engineering has been done that has been for 5.56, but that simply isn't the case.

A lot of companies jumped on the bandwagon from a "drop-in" parts modularity standpoint that the AR allows for assembly, while not looking at the real meat and potatoes of gassing, metallurgy, feed system, extraction, ejection, correct geometry for critical parts, and especially the mags.  These are all critical areas of focus that need someone with ownership over the whole shebang to bring the system to a level of maturity where it becomes a viable package for Joe and Jane consumer.

The reason Alexander Arms dropped the .221 Ghengis (5.45x39) was because of shifting charge weights lot to lot from Russian ammo imports.  You can't adjust the rifle all over the place and hope to keep up with serious deviations in the gas that runs it.  

This is why I have very specific diets even for my 5.56 blasters, mainly consisting of M193, M855, and target loads with longer pills that build pressure really well.  I don't shoot Wolf in them, or weak .223 loads.  As such, even my Model 605 clone with RLGS and 15.5" barrel cycles firmly with M193, no changes to the .094" gas port.

Alright guys, I think you've convinced me that I need a seekins adjustable gas block. Anyone want to share a link for their favorite exta power recoil spring?
I have the 1:10 SA&A barrel in hand along with a gas tube and now spare gas block.

I also think I've settled on 12.5"

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I don't have a link and haven't tried it yet but I just bought a springco combination with a 5 coil extractor spring and a red xp buffer spring on eBay for $25 shipped. Springco's are made of chrome silicon that should never have to be replaced, where as a regular ss spring needs to be replaced every 5k or so. That being said I am an armorer and I have many colt m4a1's that probably have over 10k and still function fine on the stock springs. I have heard that all future federally purchased springs will be chrome silicon so they must be pretty good. As an armorer it would be nice to have a spring that is good the life of the weapon since I never really know how many rounds are through a given weapon, we generally err on the side of caution and some good springs get tossed.

For everyone that is building 7.62x39 AR's, can you please measure your barrel's lands and grooves at the muzzle?

What is the inner diameter of the grooves, groove to groove, for example?

If it's .308", you are too tight and pressures will be significantly higher than what 7.62x39 is meant to run at.  These are .311" pills.

Ask manufacturers about this before purchasing.

Surplus Arms and Ammo confirmed that their 7.62x39 barrels are .311"
FWIW. Puts my mind at ease

Just picked up a 11.5" YHM upper that was put together by a local dealer.  Been wanting to try one on my SBR lower that normally has a 12inch 5.56 upper on it.  Only put about 12 rounds through it to function test it along with two different C Products mags I borrowed. Hope to get some more range time with it soon and try different ammo.  Only thing I don't like is it was built with a railed gas block that is lower than the receiver rail.  So I'll need to find a good low profile gas block to replace it with and then put a new hand guard on it. Is there a difference in gas block for 7.62x39 vs. 5.56?

Same gas block

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